The Minimalist Superhet Radio project.

[Geordie McBoyne]

The origins of this topic can be found on the old VRAT forum.

Today I took delivery of a Bakelite cased TRF radio set. Unfortunately although well packed the tuning dial glass is broken. I'm sure something can be done about that, a new piece of glass can be cut locally, so no problem about that. I do know that certain forum members can make replica tuning scales. I believe the cabinet of the set which the subject of this discussion is similar to the Pilot Little Maestro. It's likely this was a home constructed set.
It'll be an easy task to return this TRF receiver to working order. However, this set is the ideal candidate for completing the minimalist superhet project.  The W61 RF amplifier will be replaced by a suitable frequency changer valve, there's plenty types to decide on. For starters all the valves in this set are 6.3volt octal types. As it is now: W61 RF amplifier, 6SJ7 detector and 6V6G output. So it's out with the W61 and fit a 6K8GT. The 6SJ7 low slope AF pentode will be replaced by a 6AC7 high slope RF pentode. The 6V6G will be replaced by a 6V6GT or maybe a higher slope type.
Alternatively, replace the whole lot with miniature B7G or B9A valves. For example: 12AH8, X79 etc.
Get rid of that SenTerCel HT rectifier. Fit valveholder and use 6X5GT or the miniature 6X4.

The minimalist superhet design has been proven to be viable. Other forum members have constructed their own interpretation of this receiver concept.

Geordie McBoyne.

[Murphyv310]

Hi David.
My minimalistic superhet radio still works well, it has a ECH81, EF91, & EL95. Three waveband with Denco coils and anode bend detector. No AVC, on short wave very lively with Australia a regular station on my list.

[Mike Watterson]

Anyone any idea what audio drive is possible from triode or hexode part of the ECH81?

[Geordie McBoyne]

Only a limited amount of information for the ECH81 to hand because I'm writing this at home.
From the 1961 Mullard valve and tubes book:
Triode: Va 100. gm 2.8mA/V. u 22. ra 7.8Kohms. Ia 13.5mA.
Heptode: conversion conductance quoted. gc 0.75mA/V. ra 1.03Mohms.
Va 250. Ia 3.25mA.
The ECH81 is the successor of the ECH21 which was employed in many post-war Philips radios as the frequency changer and also employed as an IF amplifier with the triode section as the audio amplifier.
Both valves have a separate oscillator injection pin connection for G3. That pin is returned to cathode when used as an RF or IF amplifier.
One of the items donated to me by Wilfrid wiwior in France is a radio chassis in which the original side contact frequency changer valve has been replaced by an ECH81. Why would anyone want to do this?
The answer: looking through the pages of a Radio Bygones book I learn that the noise performance of the ECH81 is much better than any of it's predecessors. So it was a worthwhile valve substitution after all.

For the minimalist superhet I'm considering the Brimar developed 12AH8 for the frequency changer.
Reputed to be a very valve although the conversion conductance is not as good as the ECH81.

Geordie McBoyne.

[Geordie McBoyne]

Follow up to my last post.
The characteristics of the ECH21 are very similar to the ECH81 with the exception the former has higher Pa ratings.   Ia is 3mA @ 250V. Ig2-g4 6.2mA @ 100V. Similar to the ECH81.
The attached circuit diagram shows the ECH21 operating as an IF amplifer and the triode as the audio amplifier.  Negative bias for the triode sourced from the negative return of the HT supply. Also provides AVC delay.  Can't find any more information for the heptode functioning as an RF or IF amplifier.
This information gleaned from the 1949 Mullard Valve Book.

Geordie McBoyne.

[Mike Watterson]

Thanks
I'll try both triode and hexode with a DL96 style transformer and see can I get 100mW to 250mW, plenty with a decent 5" or 6" speaker and box. Or even two triodes as push pull and one hexode as phase invert and one as preamp. Basically you can bias any Hexode/Heptode/Octode etc as a Pentode.
The NOS Russian version ECH81 are cheap and plentiful, so a minimal superhet with all ECH81 might be interesting. A 12V only version (SLA or car) with a transistor or IC audio amp (or crystal earpiece) might be viable, 0V to +1V grid bias. An ECH81 is an ECH83 according to research of Radio Museum members.

[radioman]

Isn't the heptode section of an ECH81 variable-mu though ?
In which case I don't think it would be much use for audio... unlike the triode section which should be fine.


Andy

[Geordie McBoyne]

The ECH84 has the sharp cut-off characteristic and was used as a noise cancelled sync separator in 625 line TVs.
The pin out is the same as the vari-mu ECH81. The ECH84 can be used a frequency changer in circuits where the vari-mu characteristic is not required or is not desirable.
No doubt the ECH84 can be used for many other functions. According to the Radiomuseum the valve can be used as a modulator. If a triode section is not required consider the EH90, same pin out as the EK90/6BE6 but not vari-mu.
From the Radiomuseum: http://www.radiomuseum.org/tubes/tube_ech84.html

I don't think there should be any difficulty finding the ECH84, but there is I have a few used examples for disposal.

Geordie McBoyne.

[Murphyv310]

Hi.
I think if you look closely David the pin out is different between the 81 & 84.

[Geordie McBoyne]

Hi Trevor,
G3 and G2 & G4 pins transposed. ECH81: pin 1 G3. Pin 7 G2 & G4.
ECH84: pin 1 G2 & G4. Pin 7 G3.

Geordie MBoyne.